Vibrating screens are commonly used to sort, grade or classify particulate material, such as sand and aggregate, either in the wet or dry state.
A typical vibrating screen comprises a frame, typically defined by a pair of substantially parallel side walls interconnected by transversely extending bridging members, upon which is mounted a polyurethane screen deck having small openings or slots for water and/or undersize particles to pass through.
The frame is mounted on a chassis via resilient mountings and the frame, and thus the screen, is typically vibrated by means of a pair of counter rotating rotors defining eccentric masses driven by one or more drive motors, to impart circular or reciprocating vibratory motion to the screen. The vibration imparting rotors may be arranged to impart a resultant motion to the material from an upstream to a downstream end of the or each screen deck and/or the screen deck may be arranged at a predetermined slope and material to be graded is delivered onto an upper or upstream end of the screen, typically entrained in a flow of water, particularly if the material is also being washed. The screen is vibrated at high frequency to convey the material over the screen deck and to cause undersize material (and water if present) to pass through the openings in the screen deck, oversize material being discharged from a downstream end of the deck onto a stockpile conveyor or into a collection bay or hopper.
It is known to provide a multi-deck screening assembly to produce a number of different grades of product. A plurality of screen decks, typically two or three decks, are typically arranged one above the other, and generally parallel to each other, typically each with a downward slope from an upper receiving end to a lower discharge end at which over-sized material (relative to the screen deck concerned) can be discharged. Material of a size in excess of the size of the screening apertures of each screen deck is discharged under gravity from the lower or downstream end of the respective deck onto a respective stockpile conveyor, whereas under-sized material able to pass downwardly through the screening apertures of the respective screen deck falls under gravity onto the deck below, where the further screening action takes place, or into a collection region or sump in the case of the lowest deck.
Where four different grades are required it is normally necessary to use a triple deck screen assembly, wherein a first oversize grade is provided from a downstream end of the uppermost deck, a second grade is provided from a downstream end of the intermediate deck, a third grade is provided from a downstream end of the lowest deck while a fourth undersize grade is passes through the lowest deck to be collected in a sump therebelow.
A problem with existing triple deck screen assemblies is how to arrange each of the screen decks within the dimensional constraints of the chassis, particularly in relation to the height of the assembly, while enabling over-sized material from each deck to be delivered onto a respective stockpile conveyor.